This invention relates to a method of assembling parts to form an aircraft component and more particularly to a method that involves the use of shims. The invention also relates to an aircraft component assembled by such a method and to an assembly of shims.
The assembly of aircraft components, and especially larger components, presents various challenges. In particular the assembly may have to be carried out to very narrow tolerances but also at reasonable speed and as economically as possible. An example where such issues arise is in the assembly of a wing skin to rib feet to form a wing-box of an aircraft. In a conventional process, the wing skin is manufactured in a desired shape and is then brought into position against the sub components (rib and spars, for example) of the underlying structure of the wing box to which the wing skin is to be secured. That underlying structure has outwardly projecting rib feet on which respective parts of the inner surface of the wing skin are required to rest so that fasteners can be inserted through the wing skin and the rib feet to secure them together. The external profile of the wing skin is important in terms of the aerodynamic performance of the aircraft and a strong connection between the wing skin and the rib feet is also important in terms of the structural strength of the wing-box.
Some tolerances have to be allowed for during manufacture and consequently when the wing skin is brought in an unstressed state into position against the rib feet, it is commonly found that, whilst some rib feet are in contact with the wing skin, others are spaced from it. To eliminate the spacings, there are two approaches that are adopted. A first approach is simply to deform the wing skin by the small amount necessary to bring the other rib feet into contact with the wing skin. An approach of that kind, however, results in a distortion of the external profile of the wing skin, which may adversely affect the aerodynamic performance of the wing, and in the introduction of additional internal stresses into the wing-box, which may adversely affect the structural strength of the wing. To avoid such problems, a second approach involving altering the dimensions of the underlying structure or the wing skin may be adopted. There are various ways in which that may be done, including fettling the rib feet, adding material to the wing skin or applying a liquid shim to the rib feet but all such methods have serious disadvantages and a preferred method is therefore to make use of solid shims in the manner described below.
In the method using solid shims, the wing skin and the underlying structure of the wing-box are brought into position next to each other and gaps between the rib feet and the inner surface of the wing skin are measured. Shims are then made to fill the gaps, the shape and size of each shim being chosen according to the shape and size of the gap to be filled. In this second approach the final assembly of the wing skin and the underlying structure of the wing-box has to be deferred until after the shims have been manufactured and at that later stage, the wing skin and the underlying structure have to once again be brought into position next to each other. A process of this kind is expensive both because it requires the wing skin and the underlying structure of the wing-box to be brought into position relative to one another on two separate occasions and because it introduces a delay in the manufacturing process. Typically each of the wing skin and underlying structure are mounted on expensive jigs and they remain on those jigs throughout the steps described above. Care also has to be taken to ensure that a shim that has been manufactured to a particular size and shape to suit one particular gap is used to fill that gap and not inadvertently used to fill a different gap. Also the shims need to be handled carefully from the time of their manufacture, which may be at a remote location, until the time of their use.
It is an object of the invention to provide a method of assembling parts to form an aircraft component that involves the use of shims and that overcomes or mitigates at least some of the problems described above.